astra 1.4sxi using oil loosing oil 55 plate?

my astra 1.4sxi has started to use/loose oil I dont think its the later as there are no stains from underneath the car also the car has got a loud tapping noise when you start the vehicle from cold the car is less than 5yrs old and has only 49,000 on the clock. The car is using 2ltrs/month approx of oil. Can anyone help?